6656Views12Replies

Author Options:

Need expert engineering help to build a foot-bridge!? Answered

I want to build a foot bridge across a creek! The width of the creek is 16', but I'm going 24' long to support the footers. The bridge itself will be 4' wide. I was thinking of using "C" channel iron welded onto I-beams to hold the treated 2x6's in place. I need info on what type and size of beams, etc. If it's not too obvious I'm a complete engineering noob, but this is the kind of project that makes country living worthwhile.

Thanks in advance for your kind help!

12 Replies

user
Slim49 (author)2011-08-28

For Nice robust design,
wanted to share a roof truss building trick form the wilds of W.Va.

to make your own laminated/glued 2"x12" boards.
just
add a layer of Aluminum flashing glued in between the boards.
due to the Shear strength of the metal it becomes very rigid.
Mr Steve

Select as Best AnswerUndo Best Answer

user
Burf (author)2011-07-23

Unless you are planning on using the foot bridge to ride horses, large livestock or a Harley Fat Boy across, I would recommend using a couple of treated glu-lam wood beams for the base beams. Two 4" x 8" glu-lams spanning 24' can easily support 1200 to 1500 pounds live load.
Your savings in both money and labor will be substantially reduced and with using treated lumber, galvanized bolts and lags and periodic maintenance, it will last long enough so that a need to replace it will only become an issue for your grandchildren.

Select as Best AnswerUndo Best Answer

user
orksecurity (author)Burf2011-07-24

I didn't know manufactured/laminated lumber was available in PT. Good to know!

Select as Best AnswerUndo Best Answer

user
Burf (author)orksecurity2011-07-24

Ah, sorry, I didn't mean to imply there are pressure treated glu-lams, I was referring to something like this:

http://www.bluwood.com/product.php

To the best of my current knowledge, GL will void their warranty it their product is pressure treated. I should have been clearer in my statement.

Select as Best AnswerUndo Best Answer

user
orksecurity (author)Burf2011-07-24

Thanks. I should have said "outdoor-suitable laminated beams". I'd only seen them used within structures.

Select as Best AnswerUndo Best Answer

user
Burf (author)orksecurity2011-07-24

Yeah, sometimes it can be tricky using generic terms to define specific products. There is a tendency for brand names to become generic for all similar products.
I have used Bluwood in the past and found it to be excellent. I know there are other brands and processes but I have never used them.
Like most other treated lumber, dust masks and filters are imperative when cutting or sanding.

Select as Best AnswerUndo Best Answer

user
rickharris (author)2011-07-24

What kind of load is required

What budget do you have

What skills can you employ - Wood work - Metal work - welding etc.

How critical is this bridge i.e. how many people may die or be injured if it fails?

For my part for pedestrian traffic I would take a clue from house building. Use bricks to build 2 firm well established piers at each side of the span.

Put across that span 2 or 3 wooden beams such as you may see spanning that sort of gap in a house. say 8 x 6 inches. made from good solid knot free wood.

These sizes depend much on the distance spanned and the weight it has to carry.

Bridge these with planks to make a floor and the fit a side rail

Select as Best AnswerUndo Best Answer

user
diyoutdoorsman (author)2011-07-23

Here is a link to a site with information on constructing basic bridge trusses:

http://mysite.du.edu/~jcalvert/tech/machines/bridges.htm

Select as Best AnswerUndo Best Answer

user
orksecurity (author)iceng2011-07-23

That was sorta my own reaction -- how many cars is this intended to carry per day? It sounds like you're seriously overengineering... and in a race between metal rusting and wood rotting I'm not sure the metal wins.

Websearching for "foot bridge plans" finds both free and for-pay resources. Your proposed length is larger than most of the free plans, but it looks like some of the examples at http://www.woodworkersworkshop.com/resources/index.php?cat=420 get that large and larger. (Design drawings for a 1920's 60' covered bridge from the New Brunswick Department of Transportation?)

Select as Best AnswerUndo Best Answer